The present invention relates generally to the field of electrophotographic imaging techniques, and, more particularly, is directed to the use of a multi-layered photoresponsive foraminated structure which is capable of having stored thereon a charge pattern corresponding to the light and dark areas of a graphic original.
The construction and use of such photoconductive screens, generally known in this art as modulators, is well developed. The patent literature describes in great detail modulators which consist of two layers, namely the metal screen overcoated with a photoconductive material, as well as three- and four-layered modulators which include as separate layers, in addition to the photoconductor and the metal screen, insulating layers and other metal layers.
It has been known to impart different properties to modulators by varying the number and kind of layers that make up the screen as well as their position relative to one another in the make-up of the modulator. Two-layered screens, for example, made up of a metal screen and a photoconductor, are known to sustain a charge pattern for a brief period permitting limited repetitive use. The ability of such modulators to sustain a charge pattern is due to the dipole charge created across the photoconductive layer. This charge pattern, or charge distribution system as it is known in this art, can endure for a sufficient length of time, usually several minutes, permitting some repetitive use. Significant improvement in the properties of the member to retain the charge distribution system was achieved by the addition of a transparent insulating layer applied over the photoconductive layer. Such an arrangement permitted the creation of the charge distribution system on the surface of the insulating layer which was not affected by radiation and the resulting leakage to ground through a conductive contact. It has also been found that by changing the sequence of certain processing steps or by carrying out these steps simultaneously, surprising properties are imparted to the modulator such as dual dipole fields on the same modulator which correspond to the image and non-image areas on the graphic original.
All of the foregoing advancements in the art, which gave the unique results, were completely unpredictable and unsuspecting.
The modulator art still suffers from deficiencies because of the requirement to specially position one surface of the modulator when creating the charge distribution system on the modulator on the one hand and move it to expose a different surface when it is used for imaging the receiving sheet. Previously known structures required separate charge particle generating apparatus and illumination sources to be juxtaposed different faces of the modulator in order to permit its use in a reproduction system. This tends to complicate the apparatus calling for duplicate instrumentalities to be associated with the modulator for certain of the processing steps. In the circumstance that the modulator is moved with respect to these instrumentalities, it then requires additional apparatus to move the modulator so that the proper surface confronts the particular instrumentality.
Such prior art systems and modulators are fully described in U.S. Patent application Ser. No. 552,184 filed Feb. 24, 1975 in the name of Houshang Rasekhi and assigned to the same assignee as this invention.